In many data storage applications, there is a requirement for a disc drive having a high storage capacity within a small physical envelope. To satisfy this, Winchester type disc drives have come into common use. Such drives enclose one or more hard discs in a sealed enclosure of the type disclosed in the above 690 patent application.
In such disc drives, where a high storage capacity is contained within a limited space, it is important to provide a system in which the discs and read/write head assemblies remain precisely aligned so that the data can be reliably accessed. That is, the disc or discs must be tightly clamped to the upright hub of the spindle motor in a precise perpendicular orientation thereto. Such alignment requires precise machining of the spindle hub and that substantial clamping forces be exerted on the disc.
The problem is compounded by the current move to 31/2" disc drives. Since the available surface area for recording on the disc is being dramatically reduced, the diameter of the spindle must be minimized, as spindle shaft diameter directly subtracts from available recording surface area.
To maintain alignment, the disc must be clamped to the spindle with a considerable force. A conventional assembly method uses a single screw, screwed into the center of the shaft, to fasten a circular clamp against the disc or stack. To obtain the required clamping force without shearing the screw, this screw should be fairly large. Inserting a large screw into a minimum diameter shaft leaves a very thin spindle shaft wall adjacent the screw, providing only marginal strength in the portion of the shaft where the screw is attached.
Conventional solutions to the conflicting demands of minimizing shaft size and maximizing clamping force are (1) the use of three or six screws distributed around the periphery of the top of the hub to clamp the disc stack (the disadvantage of this approach is uneven distribution of the clamping forces over the disc, leading to potential warpage of the discs); or (2) the use of a stepped shaft with a larger diameter where the hub and single screw are placed. Unfortunately, such a stepped shaft is quite expensive to fabricate. Moreover, both of these methods involve substantially higher manufacturing and/or assembly costs than the use of a single screw in a shaft which does not have a step. Therefore, the problem to be addressed is to reliably clamp a disc in place on a motor spindle, especially in small diameter (e.g., 3.5") disc drives.